Arrange onion slices in the bottom of a 4 to 6-quart slow cooker. Trim excess fat from pork tenderloins about and 1"thickIn a small bowl, combine the mustard, vinegar, brown sugar, thyme, and garlic powder, if using. Coat the pork tenderloin pieces with the mixture and arrange over the onions. Spoon remaining honey mustard mixture over the pork. Cut the zucchini into thick slices (1/2 to 1 inch thickness) and arrange over the pork. Sprinkle with salt and pepper. Cover and cook on LOW for 6 to 8 hours, or on HIGH for 3 to 4 hours. Baste with drippings about halfway through cooking, if possible..